The present invention relates generally to the communication of packet data, such as TCP-formatted data, in a communication system which includes a radio-link, such as a UMTS (universal mobile telephone service) wireless data network. More particularly, the present invention relates to apparatus, and an associated method, by which more optimally to communicate data packets in the UMTS, or other, communication system.
Advancements in communication technologies have permitted the introduction of, and popularization of, new types of communication systems. As a result of such advancements, significant increases in the rates of data transmission, have been permitted. And, new types of communication services have also been made possible.
A radio communication system is exemplary of a type of communication system which has benefited from advancements in communication technologies. At least a portion of a communication path utilized in a radio communication system includes a radio-link. A radio communication system inherently increases communication mobility as communication channels defined in such a system are formed of radio channels and do not require wireline connections for their formation.
Advancements in digital communication techniques are amongst the advancements in communication technologies which have permitted the introduction of the new types of communication systems. Communications effectuated through the use of digital communication techniques are generally of improved bandwidth efficiencies in comparison to communications effectuated utilizing conventional, analog techniques.
A packet data communication system is also exemplary of a communication system made possible as a result of advancements in communication technologies. In a packet communication system, groups of digital bits are formatted into packets to form packets of data. The packets of data are communicated, either individually, or in groups, at discrete intervals. Once received, the packets of data are concatenated together to recreate the informational content of the digital bits of which the packets are formed.
Because packets of data can be communicated at discrete intervals, the communication channel upon which the packets are transmitted need not be dedicated to a single communication pair. Instead, a shared communication channel can be used by a plurality of communication pairs to communicate packets of data on the shared channel.
Standardized protocols by which to format and to communicate packets of data have been developed. A TCP/IP (transmission control protocol/Internet protocol) is exemplary of a packet formatting scheme. And, an X.25 protocol describes another exemplary protocol scheme. Standards relating to conventional packet communication systems have been promulgated for both conventional wireline, as well as wireless, systems.
Packet radio services have been proposed, for instance, for several different cellular communication systems. A cellular communication system is a type of radio communication system, widely implemented and popularly-used. Exemplary of such a packet radio service is the GPRS (General Packet Radio Service) system for GSM (Global System for Mobile Communications).
One proposal is for a so-called 3G (third generation) cellular communication system, referred to as a UMTS (universal mobile telecommunications system) network. Packet data communications are provided for therein. In this proposed system, as well as others, packet data is communicated between a mobile host and a network host. A communication path formed between the mobile and network hosts includes at least one radio-link formed between the mobile host and infrastructure of the UMTS network. Proposals related to the UMTS network include the use of TCP/IP protocols for end-to-end communications, viz., for communications over the wireless and also the fixed parts of the UMTS network. Such a service is typically a xe2x80x9cbest effortxe2x80x9d service, i.e., a service without a guaranteed quality of service. The infrastructure of the UMTS network includes both a wireline IP-based UMTS core network and a radio part, i.e., a radio-link, formed between the mobile host and a base station, forming a portion of the UMTS core network. TCP-based protocols have, however, conventionally been designed for conventional, wireline networks. In conventional TCP protocols, measures intended to control the flow of data and possible congestion within the communication network are designed according to the characteristics of wireline networks where packet losses are often the result of congestion. Congestion arises, for instance, because of the aforementioned sharing of communication resources for different communication pairs. When a packet communication system is implemented in wireless form, however, packet losses are often due to bit errors and/or packet losses introduced during transmission on a radio-link.
Because a UMTS network includes both a wireline, core network and also a radio part, packet losses occurring at the radio part, such as due to communication handovers or corruption on the radio-link are retransmitted locally. When the UMTS is defined in terms of logical layers, local retransmission means, for example, that data packets are retransmitted over the radio link by a radio-link control (RLC) layer. These local retransmissions decrease end-to-end throughput between the mobile and network hosts due to the time required to effectuate the local retransmissions. If a conventional TCP protocol is used in connection with a data transmission network, such as a UMTS network, implemented at least in part over a radio communication link, a sending station originating TCP data continues sending packet data at a constant rate, irrespective of the local retransmissions at the radio part of the network. Thus, the possibility for congestion of the UMTS core network increases, as new packets are transmitted from the network host in the fixed line part of the network while earlier packets are still undergoing retransmission over the radio link to recover from losses in the radio-link. Deleterious results, such as spurious time-outs of the sending station, can occur, significantly reducing the end-to-end performance of the network.
Spurious time-outs occur because of the additional time taken to receive acknowledgments for data packets that are retransmitted over the radio-link under local control of a radio link control layer (RLC). If data packets are retransmitted by the radio link control layer, additional time elapses before the sending TCP protocol in the mobile host receives an acknowledgment that a particular packet has been received by the receiving host e.g. in the fixed line part of the network. By the time an acknowledgment is received, the TCP retransmission timer in the mobile host may have already expired and conventional congestion control measures may have been initiated by the sending TCP, resulting in decreased data throughput. Furthermore, in this situation, initiation of conventional congestion control mechanisms is erroneous because the delayed acknowledgment was due to the additional time required for retransmission over the radio link, rather than real congestion in the network. According to the invention, this erroneous initiation of TCP congestion control measures is prevented by increasing TCP timer time-out values in conditions where there is an increased likelihood of retransmission over the radio link, for example in situations where there is degradation in the quality of the radio-link or a decrease in the bandwidth available for communication over the radio link. On the other hand, if true congestion of the communication network occurs, the method according to the invention still allows conventional congestion control measures to be initiated.
If a manner could be provided by which better to effectuate packet data transmission by a sending station to take into account the performance of the radio part of the system, improved system operation would result.
QoS (Quality of Service) levels are also proposed to be defined in the UMTS network. The QoS levels define, in general, performance parameters pursuant to which a particular communication service is to be effectuated. Several communication services are non-real-time services, such as communications with the WWW (World Wide Web), TELNET(trademark), e-mail services, etc. Applications to effectuate such services, logically, run on top of a TCP logical layer. And, such communication services typically are implemented at QoS levels referred to as xe2x80x9cbest-effortxe2x80x9d traffic classes. Such traffic classes do not give guarantees of available bandwidth and, hence, delivery times. Conversely, communication services which are of a real-time nature typically are implemented at higher QoS levels and such communication services are effectuated with a higher priority than non-real-time TCP-related services. Because of the lower priority levels of the TCP-related, non-real-time services, the bandwidth available to effectuate such services is susceptible to rapid change.
Conventional manners by which to effectuate TCP flow control do not include a manner by which to set transmission rates according to such rapid changes. In conventional TCP implementations, a standard mechanism, referred to as self-clocking behavior is used to limit the transmission rate of a sending station. Self-clocking behavior refers to a manner by which the sending station is able to send a new packet, responsive to reception of an acknowledgment of an earlier-transmitted packet, if the size of the transmission window remains constant. In conventional TCP operation, however, the transmission window is not constant. Instead, the transmission window is of a size which is adjusted regularly, according to the arrival of acknowledgments and occurrence of retransmission time-outs. In practice, then, in standard operation, a sending station increases a transmission window size until some point in a communication path, such as the radio access node (RAN).
Thereafter, congestion control mechanisms are implemented, but such implementations abruptly slow down transmission rates of communications. This behavior also results in reduced end-to-end throughput rates.
If a manner could be provided by which better to effectuate communication of packet data by taking into better account changes of bandwidth availabilities for the communication of the packet data, improved system operation would further result.
It is in light of this background information related to packet data communications that the significant improvements of the present invention have evolved.
The present invention, accordingly, advantageously provides apparatus and an associated method, by which more optimally to communicate data packets in a packet communication system, such as a UMTS (Universal Mobile Telecommunications System) wireless data network.
Operation of an embodiment of the present invention better optimizes the size of a transmission window within which a sending station sends a packet of data. By better selecting the size of the transmission window, the throughput rates of data communication of the packet data is improved.
Operation of a further, or alternate, embodiment of the present invention provides a manner by which to adjust a retransmission timer responsive to changes in the characteristics of a radio-link upon which data packets are communicated. The timer is adjusted in a manner to reduce the occurrence of spurious retransmissions as a result of changing radio-link conditions.
In one aspect of the present invention, apparatus is provided for a mobile station, herein referred to as a mobile host, by which to select an optimal transmission window within which a data packet is to be transmitted thereto by a network host. Determination is made at the mobile host of the optimal size of the transmission window responsive to determination of throughput rates, or other link status indication related to the radio-link. Responsive to the measured, or otherwise determined, indication, selection is made of the optimal transmission window size. A value respective of the optimal transmission window size is then sent to the network host. The optimal transmission window size is used by the network host as a maximum size of the transmission window within which the network host thereafter transmits a data packet.
In another aspect of the present invention, apparatus is provided for a mobile host to select a time-out value for a retransmission timer of the mobile host. Measurement, or other determination, of a radio-link quality indication is made. Responsive to a value of the radio-link quality being beyond a threshold value, the timer""s time out value is adjusted. In one implementation, the radio link quality indication is representative of changes in communication quality levels. For example, if a significant deterioration in the quality of the radio link is indicated, the time-out of the retransmission timer is increased. In a packet data communications network implemented at least in part over a radio link, time-out values which are too low can cause spurious time-outs. This happens because of the additional time taken to perform retransmission over the radio link, under the control of a radio link control layer, for example. As previously explained, this results in decreased throughput as congestion control procedures are started, but performance of such procedures is in vain. In a situation in which real congestion of the core network exists, the mobile host does not use an excessively large window.
In one implementation, improved TCP flow control is provided for a mobile host operable in an IP network. Improved throughput rates are made possible by determinations made at the mobile host of indications related to the TCP communications upon a radio link forming a portion of the communication path between the mobile host and a network host (or any sending TCP station). Link layer status information at a radio link control layer which specifically provides information about the radio-link is used to optimize communications. Throughput and link status data is used to set an optimal TCP offered window size. And, such data is also used to improve the ability of the retransmission timers of the mobile host to react to changes in the link status or in available bandwidth. In one embodiment, all necessary determinations and selections required for operation of the embodiment of the present invention are effectuated at the mobile host.
In a further implementation, apparatus is provided for a UMTS mobile terminal to optimize better TCP protocols to account for flow and congestion characteristics of wireless communication links. In contrast to conventional TCP flow and congestion control measures, which are specifically designed for fixed networks, operation of an embodiment of the present invention takes into account radio-link characteristics to optimize TCP transmission parameters. Data throughput and radio-link status indications, e.g., from a UMTS protocol stack, are used to set an optimal TCP window size. And, the data throughput and radio-link status indications are also used to improve the ability of TCP retransmission timers of the mobile terminal to react to changes in the link status or in available bandwidth. Implementation of the various embodiments of the present invention can be effectuated entirely at the mobile terminal, thus requiring no changes to fixed network elements, such as IP routers or network terminals.
In these and other aspects, therefore, apparatus, and an associated method, is provided for a first host operable in a communication system in which packet data is communicated between the first host and a second host upon a communication path in which the communication path includes a radio-link. The apparatus, and associated method, selects an optimal window size within which to transmit a data packet. A radio-link status determiner is coupled to receive indications of the radio-link forming a portion of the communication path between the first host and the second host. The radio-link status determiner determines an indication of a characteristic of the radio-link. The radio-link status determiner also generates a radio-link status indication indicative of the indication of the characteristic determined thereat. An optimal window size selector is coupled to receive the radio-link status indication generated by the radio-link status determiner. The optimal window size selector selects an optimal window size within which to transmit the data packet.
In these and other aspects, apparatus, and an associated method, are also provided for a communication system in which packet data is communicated between a first host and a second host upon a communication path. The communication path includes a radio-link, and the first host has a retransmission timer at least for selecting when to retransmit the data packet. Selection is made of a time-out value of a retransmission timer. A radio-link status determiner is coupled to receive indications of the radio-link forming a portion of the communication path between the first host and the second host. The radio-link status determiner determines an indication of radio-link quality of the radio-link. The radio-link status determiner also generates a radio-link quality indication indicative of the indication of the radio-link quality determined thereat. A retransmission timer time-out value selector is coupled to receive a value representative of the radio-link quality indication generated by the radio-link status determiner. The retransmission timer time-out value selector selects a time-out value of the retransmission timer. Selection is made responsive to the value representative of the radio-link quality indication.
A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings, which are briefly summarized below, the following description of the presently-preferred embodiments of the invention, and the appended claims: